A study on a sequential composite plugging removal system for water injection wells in ultra-low permeability reservoirs, Northern Shaanxi

To address the challenge of a sudden surge in water injection pressure caused by composite plugging of inorganic scale deposition, microbial biofilms and organic substances in water injectors in ultra-low permeability reservoirs in northern Shaanxi, this study innovatively proposes a mild sequential composite plugging removal technology. Through multi-scale characterization such as XRD, FTIR, and SEM-EDS, the plugging material is identified as an organic-inorganic composite structure with a ratio of 1∶9, among which the polysaccharide-protein network of SRB (Sulfate-Reducing Bacteria) biofilms wraps around inorganic scales to form a dense barrier, resulting in the failure of conventional plugging removers. Based on this, a three-stage synergistic plugging removal system is constructed and its graded plugging removal mechanism is clarified. The first stage is biofilm breakdown: 0.03% quaternary ammonium salt bactericide JY-1 and 0.4%-0.5% biofilm stripper (mass ratio of amylase to protease is 1∶1) is engaged, which breaks the physical barrier of biofilms through the enzymatic degradation of polysaccharide skeletons and protein matrices. The second stage is inorganic scale dissolution: 3.0% EDTMP chelating agent, 3% HCl, and 0.8% sodium citrate is engaged, which selectively dissolves CaCO₃/FeS and inhibits secondary precipitation. The third stage is flowback assistance and formation damage prevention: 0.4% fluorocarbon surfactant FT-1, 1% isopropanol, and 0.5% imidazoline corrosion inhibitor ML-1 is engaged, which promotes the flowback of residues by reducing interfacial tension force, while the imidazoline forms an adsorption film on the pipe walls to inhibit corrosion. Laboratory-scale experiments show that this system achieves a dissolution rate of 92.3%, a corrosion rate of 0.047 mm/a, and both the anti-swelling rate and iron removal rate exceed 84%. This system is significantly superior to the traditional mud acid (45.2%) and chelating acid (65.1%) systems in terms of dissolution rate. In field tests, the water injection pressure decreased from 15 MPa to 2.0 MPa, with an effective period of more than 6 months. This technology solves the plugging challenges of water injectors in this oil area and provides an efficient and low-damage approach for the treatment of composite plugging in ultra-low permeability oilfields.